Composition and method for removing copper containing iron oxide scales from ferrous metals

ABSTRACT

A METHOD OF AND COMPOSITION FOR REMOVING COPPER AND IRON OXIDE CONTAINING INCRUSTATIONS FROM A FERROUS METAL SURFACE SO AS TO PREVENT OR AT LEAST IMPEDE THE FORMATION OF A CURDY ADHESIVE PRECIPITATE AND UTILIZED LOW RATIOS OF COPPER COMPLEXOR-TO-COPPER. A SURFACE SOUGHT TO BE CLEANED IS CONTACTED WITH AN AQUEOUS ACID SOLUTION COMPRISING A COMPOSTE COPPER COMPLEXOR AND (II) HEXAHYDROPYRIMIDINE-2-THIONE PRESENT AS A FIRST COPPER COMPLEXOR IN A CONCENTRATION OF FROM ABOUT 80% TO ABOUT 20% BY WEIGHT OF THE MIXED COMPLEXOR, AND (II) THIOUREA PRESENT AS A SECOND COPPER COMPLEXOR IN A CONCENTRATION OF FROM ABOUT 20% TO ABOUT 80% BY WEIGHT OF THE COMPOSITE COMPLEXOR. THE COMPOSITE COPPER COMPLEXOR IS PRESENT IN SOLUTION IN A WEIGHT RATIO OF AT LEAST ABOUT 4:1 RELATIVE TO THE COPPER SOUGHT TO BE REMOVED. THE SOLUTION ALSO INCLUDES AN ACID, SELECTED FROM THE GROUP CONSISTING OF HYDROCHLORIC, SULFURIC, SULFAMIC AND PHOSPHORIC ACIDS, AND PRESENT IN A CONCENTRATION OF FROM ABOUT 3% TO ABOUT 30% BY WEIGHT.

22-149. AU 165 E 3,730,901 Patented May 1, 1973 ice accumulate on the surface of the equipment. Where these 3,730,901 deposits interfere with the efficient operation of the equip- COMPOSITION AND METHOD FOR REMOVING COPPER CONTAINING IRON OXIDE SCALES FROM FERROUS METALS ment they are removed. In recent years, it has been preferred to remove the deposits chemically; that is, chemicals which dissolve the deposits are utilized.

5 g tigg fi iggg ggg gg g' gg gs Some solvents utilized to dissolve the deposits include Okla" assign to Hambunon Company, Duncan, aqueous solutions of certain ac1ds. Thus when aqueous Okla solutions of many acids are brought into contact with fer- No Drawing. Continuation-impart of applicati n S NO, rous metal surfaces on which there is deposited copper 56,987, July 6, 1970, and Ser. No. 138,410, Apr. 29, metal and iron oxide, the iron oxide is ordinarily dissolved 1971, now abandoned, and a divisiojl 0f PP P but the copper deposit is frequently not removed from the 243,136, P 27, 1972- This Pl metal surface. Where the deposit contains an oxide of 1972' 5 35 1/08 copper, it is ordinarily dissolved by the acid, but metallic Us Cl C 3g 1 8 Claims copper will plate, i.e. redeposit, from the acidic solution on the ferrous metal surface. Thus the acid alone does not com letel remove the offensive de osit. ABSTRACT OF THE DISCLOSURE Prior publii ations such as United Stiites Martin et al. A method of and composition for removing copper and Patent 2,959,555 disclose that boiler cleaning formulairon oxide containing incrustations from a ferrous metal n5 known at least as ear y as 1945 f comprising surface so as to prevent or at least impede the formation acidic sqlutlonspf 11011113 y be used slmultanequsly of a curdy adhesive precipitate and utilize low ratios of remove o oxlde, pp and pp oxide from boilersooppor oomoloxomohoppon Such stolutionzlwotulddbe fomprised of a figst sglvent com; A surface sought to be cleaned is contacted with an Pollen Opera 9 0 1350 VB, 1011126. I 1 P an aqueous acid solution comprising a composite copper a second componenf capable of p 'f Y dissolved complexor composed of (i) hexahydropyrimidine-2-thione copper, QPP 10115, f deposllmg on fefroqs present as a fir t copper oomploxor in a oonoontratioo I metal surface 1n contact with the removal solution. This f f about to about 20% by weight f the i d second component has been referred to in the art, and will complexor, and (ii) thiourea present as a second copper be F F i a5 K pp pl f complexor in a concentration of from about 20% to about P P E forfnulmjons comatmfng hydrochlor 1C 0% by weight of the composite oomploxor, The 30 acid and thiourea which will perform this concurrent deposite copper complexor is present in solution in a weight P d15lVmg f pp complexmg fu ct on were disratio of at least about 4:1 relative to the copper sought to g gg s s lgig f S g gt t l g g tbe removed. The solution also includes an acid, selected an In 111 1116 a BS 0 HS a from the group consisting of hydrochloric, sulfuric, sul- 9 PP Solutions comifining hlfdrochlofic famic and phosphoric acids, and present in a concentraacid and thlourea the amounts destgnatedym Claim 1 of fr about 3% to about 30% by weight of Cardwell et a1. (i.e. one percent thiourea in a five to twenty-five percent hydrochloric acid solution) and designated in lines 66-72 of column 4 of Robinson (i.e. one RELATED APPLICATIONS percent thiourea in a five to twenty-five percent hydro- This pp is a continuation in p of parent p 40 chloric acid solution) perform the boiler cleaning operplication Ser. No. 56,987, filed July 6, 1970 now abanmm "Y by et doned, and incorporates the disclosure of this parent ap- Solution chcmlstry Plimaion herfiin by f T application is also a When it is desired to remove metallic copper from a ferconfinuation'm'part of aPPlliiatlon 138,410: filcd rous metal surface by a chemical solution, the solution, P 291971andn w aPandmndin addition to having copper complexing ability, should This Presem applfcatfon also compnses a dlvlslon of possess the capability to convert the copper metal to the a "concurrent" PP 248,136 filed P ionic form. This conversion capability is referred to here 27, 1972 pq Composltlon and Method f RFmOVmE in as oxidation. The oxidation capability is inherent in Copper comalnmg from Metals," deslgnafmg John the acidic solvent component of the solution and may be Knox John Smlth and Roy Stout as q y i enhanced through the intervention of an oxidizing suband assign! assignee of the Preicm appllcatlonstance which may be exteriorly added, such as free oxy- This concurrent" application discloses and claims, int er gen or which may be produced by the solution reaction alia, a method and composition wherein copper Contaln' with a portion of the deposit itself, e.g., ferric irons formed ing scale is removed, with relatively high tlefficiency, from f the f -i oxide i h scale" ferrous metal surfaces with an aqueous so ution comprising the above-described mixture of compounds. Preclpltate charactensnc pmbems In connection with the removal of the deposits, certain BACKGROUND AND SUMMARY OF INVENTION known copper complexors have been utilized individually with acidic solutions to facilitate the one-step removal General background of the copper and iron oxide from ferrous metal surfaces. This invention relates to compositions for complexing However, whenever the copper complexor-tocopper copper ions. This invention also relates to compositions weight ratio is low, many of the known copper complexors and metohds for removing copper and copper compounds form an undesirable curdy, adhesive precipitate which may from a metal surface. It further relates to compczisitionsulltimatielyhserioulszlfi' damage thef iguiiment sought to be and methods for removing copper, copper compoun s, an c eane us, a ough some 0 e nown copper comiron oxide deposits from metal surfaces. This invention still plexors may be limitedly effective in maintaining copper further relates to the simultaneous removal of iron oxide in solution, they can allow insoluble copper complexes to scales and copper containing deposits from ferrous metal be redeposited upon the equipment surface before the surfaces, such as those encountered in steam boilers. cleaning operation is finished when excessive quantities In the operation of certain types of equipment, deposits containing copper, its oxides, and iron oxide frequently of copper are present. Such redeposition is generall undesirable in industrial cleaning and can cause heat transi I I fer irregularities such as hot spots in industrial boilers.

Presently used copper complexors require .relatively high weight ratios of copper complexor-to-copper in order to satisfactorily remove copper incrustations without forming the above-mentioned undesirable precipitate. Compositions and methods have thus been sought which would permit the use of lower weights of copper complexor per unit weight of copper to be removed without the occurrence of replating and without the formation of the undesirable precipitate. Thiourea, 1,3-diethyl thiourea, ethylene thiourea, monomethyl thiourea, and monoethyl thiourea are known to possess copper complexing ability; however, these complexors tend to form the undesirable curdy, adhesive precipitates when an insufficient concentration of coppper complexor is present relative to the weight of the copper to be removed.

Other known copper complexors are hexahydropyrimidine-Z-thione, N-(2-hydroxyethyl)-ethylene thiourea, diethanol thiourea, and 4-methylimidazolidine-Z-thione. Although this latter group of copper complexors avoids the problems of replating and the formation of an undesirable precipitate, they require the use of relatively high copper complexor-to-copper weight ratios.

These and other problems are solved or at least substantially alleviated by the present invention which provides a composition and method for complexing copper IOI'lS.

SUMMARY OF INVENTION Concept of present application We have discovered a composition consisting essentially of a mixture of at least two compounds, which composition acts to complex copper ions to a degree not to be expected nor predicted from the complexing ability, when acting alone, of the individual compounds utilized in the mixture.

In another aspect we have discovered a high efficiency cleaning solution for the simultaneous removal of iron and copper deposits from metal surfaces comprising a solvent for dissolving the deposits and a composite copper complexor for preventing dissolved copper from forming new deposits on the metal surface.

In still another aspect of this invention, there is provided a high efficiency method for the simultaneous removal of copper and iron deposits from a ferrous metal surface.

The cleaning solution of this invention effectively removes iron and copper deposits from ferrous metal surfaces without forming a curdy, adhesive precipitate, and without plating of dissolved copper from the cleaning solution on the metal surface. Furthermore, the copper complexing capability of the cleaning solution is surprisingly more efficient than heretofore known solutions in that the required weight of composite copper complexor per unit weight of copper removed, is unexpectedly lower than for previously known copper complexors.

In relation to a preferred embodiment, this invention contemplates an acid cleaning solution for the simultaneous removal of copper and iron deposits from a ferrous metal surface while substantially precluding both the redeposition of copper and the formation of a curdy adhesive precipitate at low complexor-to-copper ratios. This solution comprises an aqueous acid solution selected from the group consisting of hydrochloric acid, sulfuric acid, sulfamic acid and phosphoric acid, and this acid solution has an acid concentration in the range of from about .5% to about 50% and preferably from about 3% to about 30% by weight. This solution includes hexahydropyrimidine-2-thione and thiourea as a mixed copper complexor, with the hexahydropyrimidine-Z-thione being present in a concentration of from about to about 90% and preferably from about 20% to about 80% by weight of said mixed copper complexor and the thiourea being present in a concentration of from about 80% to about 2 y we g t of said m xed pp p e or.

4 The mixed copper complexor is present in the acid solution in a ratio of at least about 2:1, preferably at least 4:1, relative to the copper sought to be complexed and up to about 5021.

Preferably this cleaning solution is constituted such that the acid solution is aqueous hydrochloric acid and contains about 5% I-ICl by weight and further comprises from about .0l% to 1% by volume of an auxiliary acid corrosion inhibitor suitable therefor as is otherwise well known in the art (see for instance United States Frost et al. Pat. 3,547,697).

In a most preferred composition, the aqueous hydrochloric acid, having an acid concentration of about 5% HC] by weight, is associated with a mixed copper complexor composed of 60% by weight hexahydropyrimidine-2-thione and 40% by weight thiourea and this mixed complexor is dissolved in the cleaning solution in a weight ratio of about 8:1 relative to copper sought to be removed.

A preferred method aspect of the invention relates to a method of removing copper and iron oxide containing incrustations from a ferrous metal surface while substantially precluding both the redeposition of copper thereon and the formation of a curdy adhesive precipitate, the method being performed at lower ratios of copper complexor-to-copper. This method comprises the contacting of the surface sought to be cleaned with an aqueous acid solution comprising, as stated above, a mixed copper complexor composed of (i) hexahydropyrimidine-Z- thione present as a first copper complexor, in a concentration preferably of from about to about 20% by weight of the mixed complexor, and (ii) thiourea present as a second copper complexor in a concentration preferably of from about 20% to about 80% by weight of the mixed complexor. As stated above, the mixed copper complexor is preferably present in the solution in a weight ratio of at least about 4:1 relative to the copper sought to be removed. As stated above, the solution used in this method also includes an acid selected from the group consisting of hydrochloric, sulfuric, sulfamic and phosphoric acids, with the acid preferably being present in a concentration of from about 3% to about 30% by weight. This method is preferably practiced with the aqueous acid solution further comprising a suitable corrosion inhibitor. Desirably, in a preferred method technique the acid is hydrochloric acid and the aqueous acid solution is agitated in the presence of the ferrous surface to be cleaned.

Broader concept of concurrent application More broadly speaking, and as is treated in the aforementioned concurrent application, the copper complexor is a composition which consists essentially of mixtures of at least two compounds represented by the general formulae:

N t t Rl Rz and R0 S R N t N In Formula 1 above, R R R and R are hydrogen, straight or branched chain alkyl radicals having in the range of 1 to 3 carbon atoms, alkenyl radicals having in the range of 2 to 3 carbon atoms or mixtures thereof. In Formula 2 above, R is a methylene group, Le. a

ens

group, having 2 t0 4 and preferably 2 to 3 ca bon atoms,

i.e. m is an integer having a value in the range of 2 to 4, preferably 2 to 3, or a group wherein n is an integer having a value in the range of to 3 and preferably 1 to 2, or a (CHz-FH-CH): C

group; and R, and R, are hydrogen, a CH CH Ol-l group or mixtures thereof.

In one preferred broad embodiment, at least one of the R R R and R groups in the above Formula 1, and at least one of the R, and R groups in the above Formula 2 are hydrogen.

In another preferred broad embodiment, each of the R, and R groups in Formula 1, and each of the R and R groups in Formula 2 is hydrogen.

Compounds believed to be useful in the practice of the broad invention which are within the scope of Formula 1 above include but are not limited to:

1,1-di-n-propyl thiourea monoisopropyl thiourea 1,3-di-isopropyl thiourea monovinyl thiourea 1,3-divinyl thiourea monoallyl thiourea mono-n-propyl thiourea 1-methyl-3-vinyl thiourea 1,3-di-n-propyl thiourea l-methyl-S-allyl thiourea From the above list of compounds, those which are currently preferred for use herein are:

thiourea monomethyl thiourea 1,3-dimethyl thiourea monoethyl thiourea 1,3-diethyl thiourea triethyl thiourea thiourea monomethyl thiourea 1,3-dimethyl thiourea monoethyl thiourea 1,3-diethyl thiourea Compounds believed to be useful herein which are within the scope of Formula 2 above include but are not limited to:

From the above list of compounds, those which are currently preferred for use herein are:

ethylene thiourea N-(2 hydroxyethyl) ethylene thiourea hexahydropyrimidine-2-thione 4-111ethylimidazolidine-Z-thione.

In the mixture of compounds of the composition all of the compounds in any given mixture can be from Formula 1, all can be from Formula 2, or some can be from Formula 1 and some from Formula 2.

The quantity of at least one single compound (and preferably two) in the composite mixed copper complexor composition of the broad invention depicted by these formulas is not greater than about parts by weight per parts by weight of the mixture, and not less than about 10 parts by weight per 100 parts by weight of the mixture, the balance being one or more other copper complexors within the parameters of the above delineated formulas. It is contemplated that such a composite or mixture shall contain at least two copper complexor compounds within the parameters of Formula 1 and/or Formula 2 and that they shall be present within the aboverecited proportions, but that other materials and compounds may be present in the solution without departing from the spirit or scope of the composition of this invention.

In the preferred embodiments of our broader invention the quantity of at least one compound in the mixture is not greater than about 80 parts by weight per 100 parts by weight of the mixture.

Accordingly, in the prefer-red embodiments of the broader invention, two-component mixtures have extreme composition limits' for a single component of about 80 to about 20 parts by weight per 100 parts by weight of the mixture. Thus, for example, in the mixture thiourea plus hexahydropyrimidine-Z-thione, thiourea is preferably present in the range of about 80 to 20 parts by weight per 100 parts by weight of the mixture and hexahydropyrimidine-Z-thione is preferably present in the range of about 20 to 80 parts by weight per 100 parts by weight of the mixture.

Further, in the preferred embodiments of the broader invention, compositions having more than two copper complexor compounds in the mixture have extreme composition limits for at least one component (and preferably two) in the range of not greater than about 80 parts by weight and not less than about 10 parts by weight per 100 parts by weight of mixture. Thus, by way of example, a three-component mixture can have 80 parts by weight of one compound and 10 parts by weight of each of two other compounds; and, in another example, a tencomponent mixture can have 10 parts by weight for each compound in the mixture.

It has been previously stated that the above-described mixed copper complexor composition of this broader invention is useful to complex copper ions. Accordingly, copper dissolved in any solution is complexed by merely adding to the copper ion-containing solution the composition of this invention. In this regard there is no known maximum quantity of copper complexor required to complex a given amount of copper except that as dictated by economics. However, it is considered that a reasonable quantity of complexor of this invention is in the range of about 2 to 50 parts by weight per one part by weight of copper to'be removed.

It is believed that the composition of this broader invention can complex copper dissolved in other than acidic solutions; however, it is preferred that the pH of the solution is, or is adjusted to about 5 or less.

As previously stated, this invention also provides an aqueous cleaning solution for the simultaneous removal of iron and copper deposits from metal, particularly ferrous metal, surfaces. The cleaning solution of this invention is comprised of a solvent for the iron and copper deposits, the mixed copper complexor composition described above, and water; the solvent is present in the cleaning solution in the range of about 0.5 to 50 percent by weight of the solution and the copper complexor of this invention is present in the cleaning solution in the range of about 0.1 to 5, preferably 0.2 to 4, and still more preferably 0.4 to 3 percent by weight of the solution with the remaining weight of the solution being substantially water.

The copper complexor utilized in the cleaning solution of this broader invention is the mixed composition described he'reinabove and reference is accordingly made to that description.

The solvent useful in the cleaning solution is any acid or acidic material capable of dissolving iron and copper deposits. Suitable acids include hydrochloric acid, hydrofluoric acid, sulfuric acid, sulfamic acid, phosphoric acid and mixtures thereof, wherein hydrochloric acid is the most preferred acid for use in the present invention. Other useful acids are citric acid, acetic acid, gluconic acid, hydroxyacetic acid, formic acid, other organic acids and mixtures thereof. The acid is preferably used in concentrations in the range of about 0.5% to about 30% by weight of the solution. Concentrations of less than about 0.5% tend to be ineffective in dissolving the copper-containing iron oxides, and concentrations above about 30% often demonstrate excessive corrosion on the ferrous surface sought to be cleaned.

Acidic materials, useful in the practice of the broader invention, in addition to the above-named acids also include acid salts such as sodium bisulfate, sodium bifiuoi'ilde, mono-sodium citrate, potassium bisulfate, and the The preferred acids and quantities in percent by weight of cleaning solution are hydrochloric acid, 3 to 10%; sulfuric acid, 5 to 15%; sulfamic acid, 3 to and phosphoric acid, 10 to 25%.

Because in some instances there is a tendency to experience corrosiveness of the acid cleaning solution with respect to the ferrous metal surface, it is frequently necessary to use a corrosion inhibitor as an optional element of the composition of the present invention. Any commercially available corrosion inhibitor may be used which is suitable for the acid selected. Such inhibitors are ordinarily used in amounts in the range of 0.01 to 1.0 percent by volume of the cleaning solution.

It has been found that an auxiliary oxidant present in the aqueous acid solution greatly increases the rate at which the copper becomes available to the complexor. It is believed the auxiliary oxidant enhances the transformation of metallic copper to ionic copper. The oxidant may be the ferric ions which occur naturally in the iron incrustations on the ferrous metal surface sought to be cleaned, oxygen in the aqueous acid solution, or any other oxidant capable of changing the elemental copper to cuprous (or possibly cupric) ions.

Mode of practice of invention It has been found that the components of the present invention do not interract chemically with each other prior to contacting the copper. However, when the complexor of this invention is used, a mixed complex is formed with copper which is different from the reaction product formed with copper when only one component is used. The mixed complex formed with copper and the complexor of this invention has been found, through X-ray crystallography, to have a different crystalline structure than the reaction product formed with copper when only one component is used.

The metal surface to be cleaned is contacted by the cleaning solution of this invention by any suitable method, e.g., soaking, pouring, spraying, circulating, and the like. The cleaning solution of this invention is particularly suitable for cleaning the inside of vessels of complex shapes where formation of a curdy, adhesive precipitate can present difficult removal problems. Normally, the area to be cleaned is contacted by filling the vessel with the cleaning solution of this invention. It is found that copper removal can be particularly enhanced by stirring or other suitable means of agitation during the contacting step.

i Durin'g'the contacting'step, the temperature of the solui n is typically maintained in the range of 50 to F., and preferably 120 to 160 F., for a period of time sufficient to dissolve the deposits. This time is generally in the range of from about 2 to 12, preferably about 4 to 8 hours. Where acids such as sulfuric acid are employed, operating temperatures beneath 150 F. may be desired to ensure that the complexors do not deteriorate.

In this connection, it has also been discovered that the corrosion rates of normally inhibited hydrochloric acid on ferrous metal surfaces sought to be cleaned are reduced when the preferred concentration of the composition of the present invention is used as compared with corrosion rates demonstrated by the acid when 100% hexahydropyrimidine-Z-thione or 100% thiourea is used.

EXAMPLES Presently preferred embodiment The presently preferred embodiment of the invention entails the use of a composite copper complexor comprising hexahydropyrimidine-Z-thione and thiourea in a solution of hydrochloric acid.

The surprising effect of using this composite copper complexor formulation is that it is synergistic, rather than additive, in its effect on the efiiciency of copper removal in pounds of copper removed per pounds of mixed complexor used. The mixed complexor also provides the features of eliminating redeposition of copper during the complexini operation and eliminating the formation of an undesirable precipitate.

The most preferred concentration of the hexahydropyrimidine-2-thione is about 60% by weight of composite complexor, with thiourea comprising the remaining 40% by weight of the composite complexor. If less than 10% by weight of composite complexor or greater than 90% by weight of composite complexor of the hexahydropyrimidine-Z-thione is present in the composition of the present invention, the significant synergistic effect on the efficiency of the composite complexor over the efficiency of its separate constituents is lost. A sharp decrease in the efficiency of the composite complexor occurs when the concentration of hexahydropyrimidinc-2-thione is decreased from about 20% by weight of composite complexor to about 10% by weight of composite complexor. A similar sharp decrease in the efficiency of the composite complexor occurs when the concentration of hexahydropyrimidine-Z-thione is increased from about to about by weight of composite complexor.

The thiourea may be present in a concentration of from about 10% to about 90% by weight of composite complexor, although the preferred range of concentration of thiourea is from about 20% to about 80% by weight of composite complexor, and the most preferred concentration is about 40% by weight of composite complexor. The significant synergistic effect of the composite complexor is lost if less than about 10% by weight of composite complexor, or greater than about 90% by weight of composite complexor, of thiourea is present in the mixture. The greatest improvement in the efiiciency occurs when the concentration of thiourea is reduced from about 90% to about 80% by weight of composite complexor, or increased from about 10% to about 20% by weight of composite complexor.

The weight ratio of mixed copper complexor-to-copper should most preferredly be at about 8:1. A complexorto-copper weight ratio of 4:1 or greater can complex copper from copper-containing iron incrustations on a ferrous surface without forming any precipitate. Concentrations of mixed complexor lower than about four parts complexor to about one part copper can result in the formation of a flocculent (i.e. dispersible) easilypumpable precipitate which is not curdy or adhesive, and is not considered harmful to most industrial equipment sought to be cleaned. There is no maximum amount of copper complexor which may be used to complex a given amount of copper except that maximum amount dictated :by ecopotnics.

To avoid turbidity in the cleaning solution, the minimum mixed complexor concentration of about five parts of mixed complexor to about one part copper for the present invention is to be compared with a minimum concentration of about nine parts complexor to one part copper for thiourea. A complexor-to-copper ratio of thiourea less than about 9:1 will result in a curdy, adhesive precipitate which is generally thought of as being undesirable. The minimum concentration of hexahydropyrimidine-2-thione which may be used prior to the formation of a precipitate is about 8.5:1, although the precipitate formed by using insufiicient amounts of hexahydropyrimidine-Z-thione is a flocculent (i.e. dispersible),

10 EXAMPLE II The following copper dissolution tests are each run in 100 ml. of 5% hydrochloric acid containing 0.2% by weight of a commercially available corrosion inhibitor at 150 F. for six hours. Each test sample contains 0.75 gram of powdered magnetite and 0.10 gram powdered copper metal. Varying weights of the aforesaid most preferred embodiment of the mixed complexor (0.60 gram, 0.80 gram, and 1.00 gram) are used in the tests. Table II, below, shows the present invention in the aforesaid "most preferred" embodiment has a greater copper complexing efficiency than either hexahydropyrimidine-Z- thione or thiourea used alone.

TABLE II Comparison of Copper Pickup by Solutions Containing Hexahydropyrimidine-2-thione,

Thiourea and Present Invention I These values, although ostensibly in excess of copper initially present, are within limits of acceptable experimental error.

easily-pumpable precipitate which is not generally considered to be harmful.

The acid is normally used in an aqueous solution with suitable concentrations being in the range of from about 3% to about 30%. Concentrations of less than about 3% may at times tend to be less than desirably effective in dissolving the copper-containing iron oxides, and concentrations above about 30% may demonstrate excessive corrosion on the ferrous surface sought to be cleaned.

Because of the corrosiveness of the incrustation, removing acid solution with respect iothe ferrous surface, it is frequently necessary to use an acid corrosion inhibitor as an optional element of the composition of the present invention. Any commercially available corrosion inhibitor may be used which is suitable for the acid selected.

The following examples are provided to further illustrate the presently preferred form of our invention, but are not to be construed to in any way limit the scope thereof. These examples are presented in the posture set forth in parent application Ser. No. 56,987.

EXAMPLE I Test procedure TABLE I complexor composition, percent Hexahydro- Complexor: Thi0- py pp urea 2-thione ratio HI) 0 8. 95 80 a0 5. 93 60 40 5. 43 40 60 6. 23 20 80 6. 46 0 100 8. 64

EXAMPLE III Test procedure TABLE III Corrosion Rates of Inhibited 5%ECltSo1utions Containing Compltxing gen 5 Concentra- Corrosion rate,

complexor tion, percent lb./sq. it./day

Present invention 1 014 Hexahydropyrimidine-Z-thione 1 020 'Ihioures 1 017 Present invention 0. 5 009 HexahydropyrimidinQ-Z-thione 0. 5 013 Thiourea 0. 5 010 EXAMPLE IV Test procedure Tests are run to determine whether the present invention is a composite of two copper complexors or a compound resulting from a chemical reaction between hexahydropyrimidine-2-thione and thiourea. Cuprous chloride dissolved in concentrated hydrochloric acid is added to solutions containing hexahydropyrimidine-Z- thions, thiourea, and the present invention. When sufii= cient copper is added to the aforesaid most preferred embodiment of the present invention so that lower coordinated complexes take precedence over the soluble higher coordinated complexes, precipitation occurs. Infrared spectra of the precipitated complexes are made. Observation of the infrared spectra indicates both hexahydropyrimidine-Z-thione and thiourea characteristics as well as modification characteristics.

11 Observations (l) A doublet at 3150 cm.- and 3350 cm." is observed (characteristic of the primary NH stretching mode of thiourea).

(2) A singlet at 330 cm.- is observed (characteristic of the secondary-Nl-IR stretching mode of hexahydropyrimidine-Z-thione).

(B) C-H stretch peaks at 2880 cm.- and 2980 cm.- are observed. Since thiourea has no C-H groups, it is believed the stretch peaks are due to the C-H groups in hexahydropyrimidine-2-thione.

('C) A broad absorption between 400 cmr and 650 cmr is observed and thought to be due to a strong NH deformation present in the primary amide group of thiourea. Hexahydropyrimidine-Z-thione has no primary amide groups.

The aforenoted infrared spectrographic data, in conjunction with the synergistic action of the mixed complexor supports the conclusion that a unique, composite, complexing action occurs.

EXAMPLE V Test procedure Solutions containing 5% inhibited hydrochloric acid and 0.52% cuprous chloride plus 1.33% magnetite are mixed with various concentrations of thiourea and hexahydropyrimidine-Z-thione and with the mixed complexor of the present invention at various ratios of thiourea-toexahydropyrimidine-Z-thione. A steel coupon is added to each solution and the solutions are maintained at 150 F. for six hours in a constant temperature bath. At the end of the test period, the solutions and steel coupons are visually inspected to determine the presence of copper plating on the steel coupon and a precipitate in the solution. Table V, below, shows the aforesaid most preferred embodiment of the present invention provides an absence of copper plating and an absence of undesirable curdy, adhesive precipitate in complexor-to-copper ratios of about 4:1 and greater.

new:

TABLE V Ratio Complexor Hexahydroconcentration, Thiopyrimidine-2- Coupon Solution weight percent urea lone condition condition 0.53 80 LP VH 0.60.--. 80 20 LP H 0.67-- 80 20 LP H 0.73.- 80 20 LP H 0.87 80 Z) LP M 0.93 80 20 VLP M 1.86.. 80 20 VLP S 2.00.... 80 20 N S 2.06."--. 80 Z) N S 2.20 80 20 N S 0.53 60 40 MP O 0.60 60 40 LP H 0.67 60 40 LP M 0.73 60 40 LP M 0.87 60 40 LP H 0.93 60 40 N H 1.86 60 40 N S 2.00 60 40 N B 2.06 60 40 N S 2. 60 40 N S 0.53 40 00 MP VH 0.60. 40 60 MP VH 0.67. 40 00 LP VH 0.73 40 60 LP H 0.87 40 60 LP H 0.93 40 60 LP H 1.86 40 60 N S 2.0!) 40 60 N S 2.06 40 60 N S 2. 40 60 N B 30 70 G 30 70 VII 30 70 V 80 70 V 30 70 V 30 70 G 30 70 E TABLE YContinued Ratio Complexor Hexahydroconcentration, Thiopyrimidine-2- Cou on Solution weight percent urea thione eon ition condition 0.53 20 MP G 0.60..-.. 20 80 MP \"H 0.67. 20 80 LP H 0.73. 20 80 \'LP H 0.87. 20 80 N H 0.93. 20 80 N S 1.86. 20 80 N S 2.00- 20 80 N S 2.06. 20 80 N S 2.20 20 80 N S 0.53 MP M 0.60. 100 MP M 0.67. 100 Mi M 0.73. 100 LP H 0.87- 100 Li H 0.93. 100 N CP 1.86. 100 N CI 2.00 100 N Cl 2.06. 100 N CI 2.20 100 N C1 Broad invention At this juncture, and in order to delineate certain broader" aspects of the invention which are treated in the aforesaid concurrent application, reference will now be made to results of a broad gauge test program which has been conducted.

In this connection, it will be recognized that the foregoing data is presented in a somewhat different format than that heretofore discussed, and may in certain instances be subject to .the normal variations and refinements involved in reorganizing and refining data.

In all of the examples which follow, the individual complexors utilied are identified by the capital letters A through I, inclusive. The following list sets out the com- 0 plexor and the letter by which it is identified:

AThiourea BMonomethyl thiourea CMonoethyl thiourea D-Hexahydropyrimidine-Z-thione E-N-(2-hydroxyethyl)-ethylene thiourea F-Ethylene thiourea G-4-methylimidazolidine-2-thione H-l,3diethyl thiourea I1,3-dimethyl thiourea EXAMPLE VI One-half gram of copper complexor is dissolved in about 85 milliliters of distilled water. The resulting copper complexor solution is titrated with an acid solution containing cuprous chloride.

The acid titrating solution is prepared by dissolving in concentrated reagent grade hydrochloric acid (37.5 parts by weight HCI per 62.5 parts by weight water) about 0.006 gram cuprous chloride per milliliter of the concentrated acid.

The titration is conducted in a 200 milliliter Berzelius tall form beaker at F. with stirring to a turbid endpoint which is reached when a thermistor temperature sensor placed in approximately the middle of the beaker within the titrated solution can no longer be seen.

Results of the titration are set out in Table IV below.

In Table VI the total weight of complexor utilized, whether singly or in admixture, is shown to be constant at 0.5 gram for each run. In those was where more than 13 one complexor is indicated, then the individual complexors are'present in each mixture in equal weights.

The above general rule, however, does not hold with respect-m runs 13, 13, 15, 16, 37, 38, 39 and 40. Accordingly, in run 12 there are 0.4 gram ((0.8)(0.5)=0.4),

14 approximately 1.5 hours in order to also form iron oxide on the inside surface of the pipe. Accordingly, the inside of each pipe is encrusted with iron oxide and copper. The weight of copper plated on each pipe and the complexorto-copper weight ratio for each solution is set out in and 0.1 gram D ((0.2(0.5)=0.1); in run 13 there are Table VIlB below.

' TABLE VII-A Solution Cleaning solution Percent Percent Percent Weight, Comploxor, H01, H40, complcxor, HCl H7O Number gms. gins. gms. gms. by 1'. by wt. by wt Each solution contains 0.48 ml. acid corrosion inhibitor which is the reaction product of a rosin amine with an aldehyde and a ire-tone plus an aeetylenic alcohol plus a polyethoxy- Iated alkyl phen 1 See Table VII-C for the spceific complexoi-(s) utilized in each run. 0.3 gram A. and 0.2 gram D; m run 15 there are 0.2 TABLE V1IB gram A, and 0.3 gram D; and in run 16 there are 0.1 C wgl q 0 e 8 I gram A, and 0.4 gram D. In runs 37, 38, 39 and 40 the 20 Solution Number plate 3 3 g g we1ghts of complexors A and D are constant at 0.135 O 24 grain and 0.2 gram respectively, and the we1ghts of com- 0.24 0:1 plexors C, F, B, and G are constant at 0.165 gram.

TABLE VI Complcxor solution Complexor Wt. ratio CuCl-HC] Total Percent by wt. Complexorzcopper R Quantity, solution, Cut, HCl, H4O, wt, No. Composition gins. ml. ems. gms. gms gms. Complexor H01 1140 Actual Predicted 0.5 14.2 0.0547 0.310 95.52 102.330 0.489 0.17 93.35 9. 0.5 10.0 0.0039 2.377 97.30 105.177 0.475 7.01 92.51 7.82 0.5 20.8 0.0801 9.244 100.41 110.154 0.454 8.39 91.15 0.2 0.5 15.1 0.0597 0.170 90.19 103.40 0.484 0.49 93.03 8.0 0.5 9.1 0.0350 4.044 91.74 90.284 0.519 4.20 95.28 14.2 0.5 18.0 0.0501 5.777 9403 100.91 0.490 5.73 93.78 9.99 0.5 18.6 0.0710 8.200 98.78 107.55 0.405 7.09 91.85 0.98 0.5 8.7 0.0335 3.800 91.45 95.810 0.522 4% 95.44 14.9 0.5 27.7 0.1000 12.310 105.53 118.34 0.423 10.40 89.18 4.9 .40 0.5 20.3 0.1013 11.088 104.43 110.008 0.429 10.02 89.55 5.3 7.05 0.5 17.2 0.0002 7.044 97.75 105.894 0.472 7.22 92.31 7.55 120 ,0.5 21.9 0.0848 9.732 101.22 111.45 0.449 8.73 90.82 5.9 9.00 0.5 23.9 0.0920 10.021 102.70 113.821 0.439 9.33 90.23 5.4 8.90 0.5 27.2 0.1407 12.088 112.20 124.788 0.400 9.00 89.91 4.77 8.85 0.5 24.8 0.0955 11.021 103.37 114.891 0.435 9.59 89.97 5.2 8.80 0.5 20.1 0.0774 8.932 99.89 109.322 0.457 7.17 91.37 0.5 8.70 0.5 22.2 0.0855 9.866 101.45 111.810 0.447 8.82 90.73 5.85 9.55 0.5 12.1 0.0400 5.377 93.79 99.847 0.501 5.39 94.11 10.73 11.05 0.5 20.5 0.1020 11.777 104.04 110.917 0.428 10.07 89.50 4.9 8.04 0.5 24.0 0.0947 10.932 103.23 114.002 0.430 9.53 00.03 5.3 7.01 0.5 22.3 0.0859 9.910 101.52 119.93 0.447 8.85 90.70 5.8 11.30 0.5 22.2 0.0855 9.866 101.45 111.810 0.447 8.82 90.73 5.85 8.21 0.5 17.5 0.0074 7.777 97.97 100.247 0.471 7.32 92.21 7.42 8.91 0.5 14.0 0.0539 0.222 95.37 102.092 0.490 0.09 93.42 9.27 11.01 0.5 21.9 0.0843 9.732 101.23 111.402 0.449 8.73 90.82 5.9 7.40 0.5 34.3 0.1321 15.243 110.42 120.103 0.390 12.08 87.52 3.78 10.55 0.5 24.4 0.0939 10.843 103.08 114.423 am 9.48 90.09 5.32 7.40 0.5 27.8 0.1070 12354 105.00 118.454 0.422 10% 89.15 407 8.10 0.5 25.0 0.098 11.377 103.97 115.847 0.432 9.82 89.75 5.07 10.20 0.5 30.5 0.1405 10.221 112.05 128.771 0.388 12.00 87.01 3.55 0.59 0.5 10.2 0.0024 7.199 97.00 104099 0.472 6.80 91.00 8.0 11.75 0.5 20.4 0.0785 9.066 100.12 109.080 0.450 8.27 91.28 0.37 12.45 0.5 35.1 0.1351 15.598 110.01 126.108 0.390 12.37 87.23 3.7 10.94 0.5 10.1 0.0020 7.155 101.10 100.755 0.400 6.58 92.90 8.1 12.10 0.5 21.0 0.0886 10.221 102.04 112.701 0.448 9.00 90.49 5.7 10.59 0.5 10.3 0.0028 7.244 97.08 104.824 0.477 0.91 92.01 8.0 8.49 0.5 27.8 0.1070 12.354 105.00 118.454 0.422 10.13 89.15 4.07 7.94 0.5 20.2 0.0778 8.977 99.97 109.447 0.457 8.20 91.34 0.4 9.19 B+A+D 0.5 20.0 0.1024 11.821 104.71 117.031 0.427 10.10 89.47 4.88 8.48 40- G+A+D 0.5 21.5 0.0828 9.555 100.93 110.935 0.451 8.01 90.94 0.04 8.20 A+H+C+D+E+F+G+H 0.5 25.9 0.0097 11.510 104.19 110.20 0.430 9.91 89.00 5.1 9.72

Table VI above clearly shows that the weight of mixed oomplexor actually required to complex copper without significant precipitate formation is unexpectedly lower than the weight which would be predicted from the complexing ability of each individual complexors in the composite when acting alone.

EXAMPLE VII The cleaning solution is maintained in each nipple at F. with gentle stirring for 5 hours after which time the solution is analyzed for dissolved copper.

The actual quantity of copper removed from the nipple by the cleaning solution is reported in Table VII-C below in terms of the weight percent of copper originally plated on the nipple. Also set out in Table VII-C, for purposes of comparison, is the weight percent of copper which would be predicted to be removed based on the removal ability of solution containing only one complexor acting alone.

In Tables VII-A and VII-C the total weight of complexor utilized, whether singly or in admixture, is shown to be constant for each solution, i.e. solution 1, 2.4 grams; solution 2, 1.44 grams; solution 3, 1.68 grams. In those runs where more than one complexor is indicated, then the individual oomplexors are present in each mixture in equal weights.

The above general rule, however, does not hold with respect to runs 11, 34, 35, 36, and 37. Accordingly, in run 11, solution 1, there are 0.96 gram Complexor A and 1.44 gram of Complexor D; and in solution 2 there are 0.576 gram of Com-plexor A and 0.864 gram of Complexor D. 5 In runs 34, 35, 36, and 37, Complexors A and D are present in constant weights in solution 1 at 0.648 gram and 0.96 gram respectively. Complexors B, C, F, and G are present in equal weights in runs 34, 35, 36, and 37 of 0.792 gram in solutions 1 and 0.4752 gram in soltuions 2.

TABLE VII-C Percent copper scale removed Solution 2 Run Solution No. Complexor composition Lactual Actual Predicted 12 6 65 39 82 43 31 35 12 4 100 B1 71 100 92 79 86 48 42.5 99 92 59 100 so 61 86 81 57 92 81 78 100 01 71 62 34.5 12).... B+F 100 66 56 20.-.- B+G 98 89 53 21.... B+H 80 82 49 C+ 99 100 86 75 59 49.5 21.... C+F 100 87 66 25.... C+G 98 90 68 26.... C-i-H 74 69 64 27.--- D+E 84 55 40.5 D+F 89 70 59 D+H 93 92 57 +G 89 93 24.5 F-i-G 88 42 41 82.... F-l-H 77 62 37 +11 98 57 39 34...- A+D+B 94 89 73.7 A+D+C 99 90 83.6

A+D+F 96 77 65.8 37..-- +D+G 97 85 67.1 E-l-F+C 52 53 29.3 39...- A+B+C+D+E+F+G+H 86 54.0

Table VII-C above clearly shows that the cleaning solutions containing a mixture of complexors generally remove surprisingly more plated copper than would be predicted from the removal ability of the individual complexors in the mixture when acting alone.

EXAMPLE VIII thione per 100 parts by weight of the mixture, are prepared by mixing the above components together in the concentrations set out in Table VIII below.

Each cleaning solution is then placed in a 2% inch nominal diameter by 4-inch long mild steel pipe nipple (ASTM A-53) having a rubber stopper in one end and being open on the other end. Each pipe is previously plated on the inside surface thereof with 0.12 gram of copper and thereafter treated in a steam atmosphere at 900 F. to 1000 F. for approximately 1.5 hours in order to also form iron oxide on the inside surface of the pipe. Accordingly, the inside of each pipe is incrusted with iron oxide and copper. The weight of mixed complexor utilized in each cleaning solution is adjusted such that the complexor-to-copper weight ratio for each solution is 10 parts by weight complexor-per 1 part by weight copper.

Runs 1 through 13 inclusive each utilize 241.2 grams of cleaning solution and runs 14 and 15 each utilize 251.25 grams of cleaning solution.

Each cleaning solution contains 0.1% by volume of the total volume of the solution of a compatible copper inhibitor.

The cleaning solution is maintained in each nipple at the temperature indicated in Table VIII with gentle stirring for 5 hours after which time the solution is analyzed for dissolved copper.

The actual quantity of copper removed from the nipple by the cleaning solution is reported in Table Vlll below in terms of the weight percent copper originally plated on the nipple.

In Example VIII which follows, the individual solvents utilized are identified by capital letters. The following list sets out the solvent and the letter by which it is identified:

I--Sodium bisulfate K-Ammonium bisulfate L-Citric acid M0.09 part by weight ammonia (NH )+1 part by weight of citric acid N-Sulfamic acid O89 parts by weight of sulfamic acid-l-6 parts by weight of citric acid P-3 parts by weight of sodium bisulfate-l-l part by weight of citric acid Q3 parts by weight of ammonium bisulfate+1 part by weight of citric acid R94 parts by weight of sodium bisulfate-l-l part by weight of citric acid S94 parts by weight of ammonium bisulfate+1 part by weight of citric acid T-Sulfuric acid, H 80 V-Phosphoric acid, H PO Z2 parts by weight hydroxyacetic acid+1 part by weight formic acid TABLE VIII Cleaning solution 7 Solvent Wt Treating Copper Percent Percent Percent ratio tempereremoved Bolutlon Compo- Quantity, Com lexor, solvent, complexor: ture, percent number sition gms. y wt. by wt. by wt copper F. by wt 11. 4 0.49 4. 73 94. 78 10:1 175 42 11. 4 0. 49 4. 73 94. 78 10. 1 175 59 11. 4 0.49 4. 73 94. 78 10:1 175 43 12.43 0.49 5.15 94.35 10:1 175 6.9 11.4 0.49 4. 73 94.78 10:1 175 77 11. 4 0. 49 4. 73 94. 78 10:1 82 11. 4 0. 49 4. 73 94. 78 10:1 175 46 11. 4 0. 49 4. 73 94. 78 10:1 175 48 R 11. 4 0. 49 4. 73 94. 78 10:1 175 50 S 11.4 0.49 4.73 94.78 10:1 175 47 T 12 0. 49 v4. 98 94. 53 10:1 89 V 24 0. 48 9. 95 89. 55 10:1 150 90 W 7. 2 0. 49 2. 99 96. 52 10:1 150 54 L 7.6 0.50 2.99 90.52 10:1 150 55 Z 7. 5 0. 50 2. 99 96. 52 10:1 150 23 17 From Table VIII it is clearly seen that the mixed complexor of this invention is useful to remove copper in the presence of a number of solvents.

SUMMARY OF ADVANTAGES An advantage of the copper complexor of the present invention is that a lower ratio of mixed complexor-tocopper can be used without the formation of any precipitate than was previously possible.

An additional advantage of the mixed copper complexor of the present invention is that it provides copper removal at a surprisingly lower ratio of complexor used to copper removed than was previously available.

Still another advantage of the mixed copper complexor of the present invention is that when insufiicient amounts of the mixed complexor are used, the precipitate formed is dispersed and easily pumpable, in contrast with the curdy, adhesive precipitate formed when insuflicient amounts of some individual thiourea derivatives are used.

Also, an advantage of the mixed copper complexor of the present invention is that, in its preferred embodiment, the complexor will not replate copper when used in a copper complexor-to-copper ratio of at least about 4: 1.

An additional advantage of the mixed copper complexor of the present invention is that is provides reduced acid corrosion of the ferrous surface sought to be treated.

These aforesaid advantages are uniquely important and significant in connection will the cleaning of industrial steam boilers.

The surprising effect of moxing the copper complexors in the aqueous acid solution is that it is synergistic rather than additive in its effect on the efiiciency of copper removal per pounds of mixed complexor used and, as noted, the mixed complexor also provides the features of substantially eliminating redeposition of copper during the complexing operation and substantially eliminating the formation of an undesirable precipitate.

With the respect to the presently preferred embodiment, it is, of course, one that both hexahydropyrimidine-2- thione and thiourea have heretofore been individually known as complexors for copper which are useful in acid descaling solutions. The use of the former for this purpose is disclosed in United States Frost et a1. Pat. 3,547,697 and the use of the latter for this purpose is disclosed in the earlier noted United States Martin et al. Pat. 2,959,555. By the same token, the foregoing examples and tables clearly and unambiguously show that these two known copper complexors, in combination, exhibit a quite pronounced and unexpected synergism.

For instance, the data in Table I show that the mixed copper complexor of the present composition containing, for instance, 80% thiourea and 20% hexahydropyrimidine-Z-thione is capable of holding an amount of copper in solution which corresponds to a complexor: copper ratio of 5.93; a mixed copper complexor containing 40% thiourea and 60% hexahydropyrimidine-Z-thione is capable of holding copper in solution in an amount corresponding to a complexor: copper ratio of 5.23; and a mixed complexor composition containing 20% thiourea and 80% hexahydropyrimidine-Z-thione is capable of holding copper in solution in an amount corresponding to a complexor: copper ratio of 6.46. By contrast, when thiourea is used as'the sole copper complexor in the solution, the amount of copper which it holds in solution corresponds to a complexor: copper ratio of 8.95, ie, the amount of complexor required to hold a given amount of copper in solution is from about 38.5 to 71% greater when thiourea is used as the sole complexor than when any oft he mixed complexors listed in Table I are used. Similarly, it can be shown that when hexahydropyrimidine-Z-thione is used as a sole copper complexor, the amount of this copper complexor which is required to hold a given quantity of copper in solution is from 34% to 65% greater than any of the mixed copper complexor compositions listed in Table I. In fact, the Table I data,

when extrapolated, wil show that even when a mixed copper complexor is used containing only 10% thiourea and hexahydropyrimidine-2-thione or 90% thiourea and 10% hexahydropyrimidine-Z-thione, about 7.5 parts of such a mixed complex is capable of holding as much copper in solution as 8.9 parts of a composition wherein thiourea is a sole complexor or as 8.64 parts of a composition wherein hexahydropyrimidine-2-thione is the sole complexor. In other words, as such an extrapolation will show, even when a mixed complexor is used containing only 10% of thiourea and 90% hexahydropyrimidine-Z-thione (and vice versa), it is more efiicient than either of its components individually in that about 23.5% more hexahydropyrimidine-Z-thione or about 28.5% more thiourea is required to hold the same amount of copper in solution than when a mixed complexor containing 10% of one and 90% of the other copper complexor is used.

The data in Example II further substantiates the various unexpected differences which make the mixed complexor so greatly superior to either of its known com ponents individually. In this connection it is significant to observe with reference to the data shown in Table II that the superiority of the claimed mixed complexor over its individual components is particularly pronounced at the lower complexor:copper ratios.

The important practical significance of this set of data is that even if a relatively high proportion of complexor is used in the descaling solution relative to the total amount of copper which is estimated to be in the scale to be removed, normally such cooper is not evenly distributed throughout the system but tends to be concentrated in certain areas. Consequently, even though the overall complexor:copper ratio is relatively high and theoretically sufiicient to hold all the copper in solution, in this areas where the copper deposits are concentrated the local ratio of complexor-to-copper will be significantly lower. In the case of a relatively inefiicient complexor such as thiourea alone this then increases the risk that some of the dissolved copper will be redeposited as insoluble copper complexes on the cleaned steam generator surface and ultimately lead to failure due to resulting hot spots during the course of steam generation.

Furthermore, as is indicated by the data in Example IV, when the mixed copper complexor of the presently preferred embodiment sequesters dissolved copper ions, it forms a mixed copper complex which is essentially different from the simple type of copper complex which is formed when either thiourea alone or hexahydropyrimidine-Z-thione alone is used as the copper complexor. In other words, the mixed copper complexor of this invention operates by a chemical mechanism which is distinct from the simple reactions of either of its components and produces an important improvement in complexor efficiency.

To sum up, the present invention departs from the teachings of patents such as Frost et al, or Martin et al because (a) the claimed invention involves the use of difierent means (a composite complexor comprising interacting hexahydropyrimidine-Z-thione and thiourea, as opposed to the prior use of either of these two complexors individually),

(b) the invention operates by a difierent mechanism by forming copper complexes which are different from and more soluble than the complexes formed by either of these two complexors individually, and

(c) the invention leads to a significantly different resuit in that substantially lower concentrations of the composite copper complexor are required than in the case of either its components, the difference being attributable to anunexpected synergism between the two components.

Thus, in light of this phenomena and the foregoing discussion, it will be clear that, while complexing copper in acid solution, the mixed complexor or mixture of this invention does not entail a mere simple additive formulation. Rather, while acting in solution to complex copper, this mixed" complexor functions as a uniquely composite, somehow interacting formulation, which yields results not attainable by any individual complexor used as a raw solution ingredient.

Having described our invention that which is claimed 1. An acid cleaning solution for the simultaneous removal of copper and iron deposits from a ferrous metal surface while precluding redeposition of copper and the formation of a curdy adhesive precipitate at low complexor-to-copper ratios comprising:

(a) an aqueous acid solution selected from the group consisting of hydrochloric acid, sulfuric acid, sulfamic acid and phosphoric acid, said acid solution having an acid concentration in the range of from about 3% to about 30% by weight, and

(b) hexahydropyrimidine-Z-thione and thiourea as a mixed copper complexor, said hexahydropyrimidine- 2-thione being present in a concentration of from about 20% to about 80% by weight of said mixed copper complexor and said thiourea being present in a concentration of from about 80% to about 20% by weight of said mixed copper complexor, and said mixed copper complexor being present in said acid solution in a ratio of at least about 4:1 relative to the copper sought to be complexed.

2. The cleaning solution of claim 1 wherein the acid solution is aqueous hydrochloric acid containing about 5% HCl by weight and which further comprises an acid corrosion inhibitor suitable therefor.

3. An acid cleaning solution for the simultaneous removal of copper and iron deposits from a ferrous metal surface while precluding redeposition of copper and the formation of a curdy adhesive precipitate at low complexor-to-copper ratios consisting essentially of:

aqueous hydrochloric acid containing about 5% HCl by weight and hexahydropyrimidine-2-thione and thiourea as a mixed copper complexor dissolved therein, said mixed copper complexor being composed of 60% by weight hexahydropyrimidine-Z-thione and 40% by weight thiourea and being dissolved in said cleaning solution in a weight ratio of about 8:1 relative to copper sought to be removed.

4. The acid cleaning solution of claim 3 further comprising a corrosion inhibitor.

5. The method of removing copper and iron oxide containing incrustations from a ferrous metal surface while precluding redeposition of copper thereon and the formation of a curdy adhesive precipitate at low ratios of copper complexor-to-copper comprising the steps of:

contacting the surface sought to be cleaned with an aqueous acid solution comprising (a) a mixed copper complexor composed of (i) hexahydropyrimidine-Z-thione as a first copper complexor, said hexahydropyrimidine-Z-thione being present in a concentration of from about 80% to about 20% by weight of said mixed complexor, and (ii) thiourea as a second copper complexor, said thiourea being present in a concentration of from about 20% to about 80% by weight of said mixed complexor, said mixed copper complexor being present in said solution in a weight ratio of at least about 4:1 relative to the copper sought to be removed, and

(b) an acid selected from the group consisting of hydrochloric, sulfuric, sulfamic and phosphoric acids, said acid being present in a concentration of from about 3% to about 30% by weight.

6. The method of claim 5 wherein said aqueous acid solution further comprises a suitable corrosion inhibitor.

7. The method of claim 5 wherein said acid is hydrochloric acid and wherein the method comprises the further step of agitating said aqueous acid solution in the presence of the ferrous surface to be cleaned.

8. The method of removing copper containing iron oxide incrustations from a ferrous surface while precluding the redeposition of copper on the surface sought to be cleaned and the formation of a curdy adhesive precipitate, consisting essentially of the steps of contacting the surface sought to be. cleaned with an aqueous acid solution comprising (a) a mixed copper complexor of (i) hexahydropyrimidine-Z-thione present in a concentration of about by weight of said mixed complexor and (ii) thiourea present in a concentration of about 40% by weight of said mixed complexor, and (b) hydrochloric acid containing about 5% RC1 by weight, and (c) said mixed complexor being present in said solution in a weight ratio of about 8:1 relative to the copper sought to be removed; and agitating said mixed copper complexor in the presence of ferrous surface sought to be cleaned.

References Cited UNITED STATES PATENTS GEORGE F. LESMES, Primary Examiner J. R. MILLER, Assistant Examiner US. Cl. X.R. 

